A CCPP (combined cycle power plant) is a power plant in combination of a gas turbine and a steam turbine with a high thermal efficiency so constructed as to lead high temperature exhaust gas from the gas turbine to a heat recovery steam generator (HRSG) and to generate steam by heat energy retained in the exhaust gas. This steam enables the power generation by the steam turbine, and coupled with power generated by the gas turbine, it is possible to improve thermal efficiency equivalent to thermal energy retained in exhaust gas compared with the independent power generation by a gas turbine.
For improving thermal efficiency of a CCPP, it is most effective to increase the hot gas temperature at the inlet of gas turbine to a higher temperature. However, even with the latest material and combustion technology, the hot gas temperatures are limited due to life time and emission reasons. To further increase the efficiency of CCPPs fuel gas preheating has been proposed.
The EP0931911 A2 describes the extraction of high pressure feed water upstream of a high pressure drum to preheat the fuel gas. The cold HP water is then sub-cooled and discharged to the main condenser.
The use of such fuel gas preheating systems improves the overall efficiency. However, it incurs mayor energy losses because high grade heat from a high pressure level is used.
The DE 10 2007 054 467 A1 describes a stepwise preheating using different heat sources from the intermediate pressure and low pressure level of the water steam cycle. After extracting heat the water used for preheating is returned to the low pressure system of the HRSG. The system described in DE 10 2007 054 467 A1 can lead to improved efficiency but is complex and expensive. In addition all return streams lead to the low pressure system thereby incurring corresponding losses.